CoreData NSPredicate with two one-to-many relations

Question

I have a CoreData model data structure like this for restricting information based on the group a user belongs to:

Category <->> Information <->> Groups.

I have an NSSet of UserGroups objects. I want to be able to filter categories based on the NSSet of Group objects, such that if a category does not contain any pieces of information which have any groups inside my NSSet, they will not be returned by my predicate.

For information I can do

[NSPredicate predicateWithFormat:@"ANY groups in (%@)",groups];

For categories I've tried the following with only a crash:

[NSPredicate predicateWithFormat:@"ANY information.groups in (%@)",groups];

But I need to write a predicate at the Category level. I am programming under the assumption that the information in my data set is sufficiently large that I cannot pull them all of them out and process them to find my Categories. I want to create predicate that will fetch only the categories that are relevant to the user based on his/her groups.

Thanks for your help!

Answer 1

The following predicate on Category should work (assuming that information is the to-many relationship from Category to Information):

[NSPredicate predicateWithFormat:@"SUBQUERY(information, $i, ANY $i.groups in %@).@count > 0",groups];

Alternatively, you could use the inverse relationships:

 // your set of Group objects:
 NSSet *groups = ...;
 // all Information objects that are related to any group object:
 NSSet *information = [groups valueForKey:@"information"] ;
 // all Category objects that are related to any information object:
 NSSet *categories = [information valueForKey:@"category"];

which can be combined to

 NSSet *categories = [groups valueForKeyPath:@"information.category"];

A disadvantage of this alternative solution might be that it creates also the intermediate set of groups in memory.